0:06Skip to 0 minutes and 6 secondsPROFESSOR RACHEL MILLS: So the next question is, where does the salt go? We can define the concept of residence time, which is essentially how much time there is between something being supplied to the ocean and it being removed from the ocean. And so you can calculate the residence time, for example, for the sodium and chloride in the ocean. And you will find that it's in excess of something like 100 million years. So there's 100 million years between being supplied by rivers to the ocean and being removed to the sediments. But the oceans themselves are something like 4 billion years old. So there's a huge amount of geological time for the salt to go round and round the system.

0:41Skip to 0 minutes and 41 secondsAnd the oceans are not saturated with salt because it's being removed via some process. Now it turns out the process for removing salt from the oceans is very like what the Romans were doing to produce salt that we put on our food. It is essentially evaporation of seawater in hot, arid parts of the ocean to form what we call evaporite deposits, which essentially suck the salt out of the ocean and keep it from building up to high levels. So if you calculate the residence time for the minor components in seawater, you'll find that they're much shorter, maybe only tens of millions of years.

1:11Skip to 1 minute and 11 secondsAnd this means that something must be removing the minor components much more effectively, much more quickly, and therefore they don't build up to the high levels like sodium chloride. So for example, organisms will precipitate calcium carbonate skeletons. And these calcium carbonate skeletons will then sink through the water column and form sediments on the sea floor. And these sediments ultimately will become the limestone deposits we see on land. So this effectively keeps the calcium and carbonate content of the ocean well below that of the sodium chloride content. So one of the major processes that removes salt from seawater is hydrothermal circulation through young ocean crusts. This is a black smoker activity we saw with Jon and Verity last week.

1:51Skip to 1 minute and 51 secondsSo cold seawater will penetrate into the sea floor through the cracks and fissures at the sea floor, driven by that magma supply at depth, where the temperatures are incredibly high. So as the seawater penetrates down into the rock, it gets hotter and hotter. And chemical reactions will be driven by the temperature at gradient and by the high pressure underneath the crust. So the first reaction that happens is the magnesium, that major element in seawater, is stripped out to form clay minerals in the crust. So magnesium is stripped from seawater. Following this, we get removal of the sulphate from seawater, which precipitates as a white mineral, which we call anhydrite in the ocean crust.

2:30Skip to 2 minutes and 30 secondsSo the chemical composition of our fluid is changing. The fluid gets more acidic as it strips protons from the rocks. In turn, it becomes more corrosive towards the rock. And metals are stripped out and sulphides are dissolved from the rock. So the fluid stinks of hydrogen sulphide. So as that fluid gets close to the magma supply, it gets hotter and hotter and hotter, maybe up to 400 degrees Celsius. At that point, it's very close to boiling. And it becomes very buoyant and is forced back up through the crust to the sea floor, where it erupts as the black smokers.

3:05Skip to 3 minutes and 5 secondsWe've seen these vents that gush out at metres per second on the sea floor, transporting with them metals and sulphide from the crust into the ocean and forming the chimney features that we call black smokers.

Where does the salt go?

So now that you have considered the vast quantities of salt that are present in the ocean, let’s get back to why that is.

As you saw in previous videos, the inputs of water to the ocean have a very different composition and only contain a small amount of salt. Rain water and rivers carry small amounts of dissolved salts to the ocean where they are mixed into the vast ocean by the global circulation that we looked at in week 2. This has been going on for billions of years.
In this video, Rachel introduces the concept of residence time: the amount of time that a chemical component spends in the ocean before being removed from it. Many river components are removed relatively quickly by biological organisms and other processes, leaving the various salts to build up over time as they are removed more slowly. Rachel also explains how hydrothermal vents are responsible for one of the major process which modifies the content of seawater.

Salt is often thought of as sodium and chloride, but only because it is the most abundant type of salt in the ocean. In fact, there are many types of salt in seawater, including chlorides, sulphates and carbonates. The mineral “halite” (sodium chloride) is often the first to precipitate when seawater evaporates because it is so highly concentrated. But many other minerals, for example, “sylvite” (potassium chloride) and “gypsum” (calcium sulphate), will also precipitate from seawater. We see this in the geological record as mineral deposits on land where oceans once existed but have since dried up entirely and left their salts behind. Here’s a link to some impressive looking salt deposits of all shapes and sizes - enjoy!